The production of hydrogen peroxide by direct synthesis from gas mixtures containing hydrogen and oxygen, by reacting the gas mixture in the presence of a liquid aqueous, aqueous-organic or organic reaction medium on a noble metal catalyst, is known. The hydrogen peroxide solutions obtainable by the direct synthesis process are of interest as oxidising agents for the catalytic oxidation of organic compounds.
In the direct synthesis of hydrogen peroxide, the undesired catalytic activity of the noble metal catalysts used in the catalytic breakdown of hydrogen peroxide to water and oxygen represents a problem. This undesired catalyst activity can be inhibited by adding a strong acid and a halide to the liquid reaction medium in an adequate concentration. However, the addition of acid and halide that is needed to obtain a high hydrogen peroxide selectivity makes the liquid reaction medium highly corrosive to metallic materials, in particular to stainless steel. Furthermore, if the hydrogen peroxide solution is used as an oxidising agent for the catalytic oxidation of organic compounds, the content of acid in particular leads to undesired secondary reactions and consecutive reactions.
One approach to solving the problem is the use of strongly acid catalyst supports, such as acid and superacid metal oxides, known from EP 504 741, activated carbon with sulfonic acid groups, known from EP 978 316, or ion-exchange resins with acid groups, known from EP 1 344 747. However, noble metal catalysts on strongly acid catalyst supports have the practical disadvantage that in the production of hydrogen peroxide the catalyst rapidly loses much of its catalytic activity.
There is therefore also a need for improved processes for the direct synthesis of hydrogen peroxide, which ensure a high selectivity of hydrogen peroxide formation with constantly high catalyst activity even without the addition of acid or with only low acid concentrations.